In recent years, liquid crystal display devices, for example, which are thinner and lighter among other characteristics than conventional cathode-ray tubes are in wide use as flat panel displays for liquid crystal televisions, monitors, portable phones, and the like. Among such liquid crystal display devices, those using an active matrix substrate for a liquid crystal panel serving as the display panel are known. Such active matrix substrates include a plurality of data wirings (source electrode wirings) and a plurality of scan wirings (gate electrode wirings) arranged in a matrix, and pixels disposed near the intersections of the data wirings and the scan wirings, each pixel having a switching element such as a thin film transistor (hereinafter simply referred to as “TFT”) and a pixel electrode connected to the switching element.
In general, in the active matrix substrate described above, thin film transistors for peripheral circuits are integrally provided in addition to those used as switching elements for pixel driving. Further, for active matrix substrates used in liquid crystal display devices equipped with a touch panel or in liquid crystal display devices equipped with illuminance sensors (ambient sensors), it has been proposed to integrally provide photodiodes (thin film diode: TFD) as optical sensors in addition to thin film transistors for pixel driving and for peripheral circuits. Thus, active matrix substrates use semiconductor devices equipped with a plurality of thin film transistors and photodiodes.
In recent years, to meet the demand for lower power consumption feature of the above-mentioned liquid crystal panels with built-in optical sensors and liquid crystal panels with built-in pixel memories, for example, a need for leakage current reduction of thin film transistors (transistors) of the semiconductor devices described above is beginning to be recognized. A known structure for suppressing the leakage current of transistors employed in conventional semiconductor devices is the LDD structure, where a low concentration impurity region (LDD region: Lightly Doped Drain) having a higher resistance than the source region or the drain region is disposed at least either between the channel region and the source region or between the channel region and the drain region.
Also, as described in Patent Document 1 below, for example, in the case of conventional semiconductor devices, it has been considered as possible to connect a plurality of (two, for example) thin film transistors (switching units) in series, i.e., first and second thin film transistors, and to connect a storage capacitance (capacitance) to the connecting section of the first and second thin film transistors to set the source-drain voltage of the second thin film transistor, to which the liquid crystal capacitance of the liquid crystal panel is connected, to almost 0V and to make the leakage current of the second thin film transistor very small. Also, it has been considered that such a conventional semiconductor device can suppress the pixel voltage fluctuation at the liquid crystal capacitance. Also, regarding this conventional semiconductor device, it has been proposed to further suppress the fluctuation of the pixel voltage, which voltage is at one end of the first and second thin film transistors, by connecting another storage capacitance to the second thin film transistor in parallel to the liquid crystal capacitance.